1. Field of the Invention
The invention relates to a circuit for acquisition of the capacitance or capacitance change of a capacitive circuit element or component, with a clock generator, a changeover contact controlled by the clock generator, a storage capacitor, a voltage source, and an evaluation stage in which one electrode of the capacitive circuit element or component is joined to the input of the changeover contact, the first output of the changeover contact is joined to the first electrode of the storage capacitor, the first electrode of the storage capacitor on the one hand is connected via a resistor network to the voltage source and on the other hand to the evaluation stage, and the second electrode of the storage capacitor is connected to a reference potential.
Within the framework of the invention, the "capacitance" is the capacitance value of a circuit element or component; a "capacitance change" is, consequently, the change of the capacitance value of a capacitive circuit element or component; the "acquisition" of capacitance or a capacitance change is both only a qualitative acquisition and also quantitative acquisition, therefore a genuine measurement; a "capacitive circuit element or component" is each circuit element and each component which has capacitive properties, and is often also called a capacitance, then the capacitance value not being intended; and a capacitive "circuit element or component" is especially a capacitor. However, within the framework of the invention, a "capacitive circuit element or component" is also the electrode of a proximity switch in cooperation with an influencing body, and for example, within the framework of the invention, a "capacitive circuit element or component" is also the capacitance which is represented by lines acting capacitively with one another. In the following, instead of "capacitive circuit element or component," a sensor capacitor is always addressed without being associated with a limitation to a capacitor in a narrower sense.
Within the framework of the invention a "voltage source" is both a voltage source overall and also a terminal of this voltage source.
Finally, it is also noted that, by way of explanation, within the framework of the invention, "changeover contact" means a switch which is often also called a two-way contact and which, therefore, also has one input and two outputs, the input being joined either to the first output or to the second output.
The subject matter of the invention is not only a circuit for acquisition of the capacitance or capacitance change of a capacitive circuit element or component, but the subject matter of the invention is also a process for acquisition of the capacitance or capacitance change of a capacitive circuit element or component.
2. Description of Related Art
A circuit of the type described initially is known from published German Patent Application No. 40 39 006, but the circuit described therein is neither intended nor suited for use in a capacitive proximity switch. Rather, this circuit is used as a capacitance-frequency converter for generating an essentially rectangular converter output signal with a frequency that is dependent on the capacitance of a capacitor to be measured. For this reason, the circuit known from German Patent Application No. 40 39 006 delivers a frequency as the output signal as well.
In circuits of the initially described type, especially in the area of the sensor capacitor, a plurality of parasitic capacitances occur which can adulterate the measurement result. Basically, these parasitic capacitances can only affect the measurement result when a voltage excursion occurs and the amount of charge which has changed or been recorded in doing so has effects on the measurement result. If this voltage excursion is prevented, the parasitic capacitances inherent in the circuit can no longer have an effect.